GLOBE Equipment - Atmosphere
GLOBE Equipment - Atmosphere
GLOBE Instrument Specifications
All GLOBE instrument specifications described below represent the minimum specifications necessary to collect scientifically valid data. GLOBE schools may use instruments that meet or exceed these specifications. For example, the GLOBE specifications for pH paper call for a range of 2 to 9 pH units. A pH paper with a range of 1 to 14 exceeds specifications and may be used by GLOBE schools.
The GLOBE sun photometer has two optical/electronic channels, one with an effective aerosol optical thickness wavelength of 505 nm and the other with an effective AOT wavelength of 625 nm, where “effective aerosol optical thickness wavelength” is defined in Brooks, David R., and Forrest M. Mims III: Development of an inexpensive handheld LED-based Sun photometer for the GLOBE program. J. Geophys. Res. 106(D5), 4733-4740, 2001. (That is, the algorithms presented in this paper are an integral part of the instrument specification.) The LED detectors for each channel must be obtained directly from the GLOBE Aerosols Science Team.
The detectors and their associated battery-powered electronics are housed in an enclosed plastic or metal box approximately 15 cm long by 5 cm high by 8 cm wide. The detectors must be mounted in a plane such that the LED chips themselves (embedded in a standard T-1-3/4 epoxy housing) are 12.5 cm from one end of the case and that end must contain a 5.5 mm (7/32") diameter sun aperture hole. The round end of the LED housing, which acts as a lens in usual LED applications, must be flattened and polished. There must be a clear line of site from this aperture hole to each detector. No internal light baffling is required.
Sunlight is aligned on the detectors through the use of two alignment brackets mounted on the outside of the case. Sunlight passes through a round hole in the front bracket and then shines upon two alignment marks on the rear bracket (one for each channel). When the sunlight spot is centered over an alignment mark, it should also be centered over the LED for the corresponding channel. (Alternate means of aligning the sun on the detectors are acceptable.)
The electronics consist of two low-power op-amp-based transimpedance amplifiers (or their functional equivalents) to convert the LED current to a voltage on the order of 1-2V in full sunlight. Noise, gain, temperature drift, and other op amp performance characteristics should be similar to that of Linear Technology LTC1050 or LTC1051 op amps. (Generic 741-type op amps or their dual equivalents are not suitable for this instrument.) Bypass capacitors should be included in the resistive feedback loops to prevent self-oscillation.
The sun photometer’s output should be monitored either by attaching an external digital voltmeter to pin jacks mounted on the case, or through a built-in digital meter. A built-in meter should display at least three digits to the right of the decimal point for output in the 1-2V range.
A digital voltmeter (or multimeter) with a DC volts setting that is either: (i) auto-ranging within the range 0-20VDC or (ii) manually selectable for range settings of 0-2VDC and 0-20VDC. For inputs of less than 10VDC (that is, up to 9.999V), the meter must display three digits to the right of the decimal point.
Digital max/min thermometers may be used. These must have either an accuracy of ± 0.5 ºC or a precision of at least ± 0.5 ºC and an error offset that is temperature independent. These thermometers can either be digital single-day max/min thermometers that are checked and reset each day or digital multi-day max/min thermometers that log temperature values for multiple days.
Digital multi-day max/min thermometers must be able to record max/min temperatures over 24-periods that can be set to begin and end within one hour of local solar noon.
Digital temperature sensors may also be used to monitor temperature. These must have either an accuracy of ± 0.5 ºC or a precision of at least ± 0.5 ºC and an error offset that is temperature independent.
The maximum/minimum thermometer will be calibrated with a second thermometer which is an organic liquid-filled thermometer with a temperature range of -5 ˚C to 50 ˚C. The thermometer must be factory calibrated and tested with standards traceable to N.I.S.T (The National Institute of Standards and Technology - United States) to an accuracy of +0.5 ˚C, with 0.5 ˚C scale divisions. It must be supplied with a protective jacket with holes at the bulb end to allow for circulation and a hole at the top by which to hang the thermometer in the instrument shelter for calibration of the maximum/minimum thermometer.
An instrument shelter is required to house the maximum/minimum thermometer and the calibration thermometer to assure scientifically usable air temperature measurements. The instrument shelter must be constructed of a material with a thermal insulation value which equals or exceeds that of seasoned white pine wood (approximately 2.0 cm thick). It must be painted white with exterior grade paint. The shelter must be vented, and be large enough to allow air circulation around the thermometer. The inside dimensions must be at least 45 cm high, 24.0 cm wide, and 12.0 cm deep. The shelter must have a hinged door on the front, be louvered on the front and sides, and have holes in the bottom and holes at the uppermost part of the sides to increase ventilation if the louvers do not extend to the top of the sides. The door must contain a lock. The instrument shelter must be mountable onto a wall or post. The top of the shelter must slope downward toward the front. The parts of the shelter must be securely fastened to each other, either using screws or with nails and glue. Joints must be sealed with weather resistant caulking compound. Detailed instructions on constructing an instrument shelter are provided in the Instrument Construction: Instrument Shelter in the Atmosphere Investigation.
Alternative building instructions can be found here: http://www.instesre.org/StevensonScreen.htm
The aneroid barometer must have a clear scale with a pressure range between 940 and 1060 millibars. The scale should be readable to the nearest whole millibar and have an accuracy of 3.5 millibars over its entire range. A set needle should be on the face of the barometer. The barometer must be calibratable. This barometer will be most useful for stations whose elevation is less than 500 meters above sea level. Schools at higher elevations will need to use an altimeter.
An altimeter is a special type of aneroid barometer designed to provide heights (using standard temperature and pressure values), as well as true atmospheric pressure readings. The scale must be given in millibars and extend from 650 millibars to 1050 millibars. Accuracy must be 3.5 millibars over the range of the instrument. The altimeter must be calibratable. This instrument is for the measurement of atmospheric pressure at elevations over 500 m.
Barometric pressure values may also be collected with a digital barometric pressure sensor. This sensor must have a pressure range of between 940 and 1060 mbars with one mbar resolution and an accuracy of 3.5 mbars over its entire range. Barometric pressures reported from the sensor must be station pressures.
The GLOBE cloud chart shall display at least one visual example of each of the 10 basic cloud types: cirrus, cirrostratus, cirrocumulus, altostratus, altocumulus, cumulus, nimbostratus, stratus, cumulonimbus, stratocumulus. Cloud cover will be visually estimated (See the Estimating Cloud Cover Learning Activity). The GLOBE cloud chart is available in multiple formats and in multiple languages and can be downloaded from the GLOBE website.
The GLOBE contrailchart shall display at least one visual example of each of the 3 contrail types: short-lived, persistent, and persistent spreading. Contrail cover will be visually estimated (See the Estimating Cloud Cover Learning Activity). The GLOBE contrail chart is available in multiple formats and in multiple languages and can be downloaded from the GLOBE website.
Precipitation will be measured with a clear view plastic rain gauge with a collector that is at least 102 mm in diameter. The rain gauge must be at least 280 mm in height with a scale indicating rain collected of 0.2 mm or less on an inner clear cylinder. It must have the capacity to measure rainfall of 280 mm without overflowing. The shape of the outer part must also be cylindrical, and overflow from the inner cylinder shall be directed to the outer part of the rain gauge. The outer cylinder must be capable of being used in the inverted position to gather a snow sample for measurement of the water content of snow. The rain gauge must be provided with the necessary hardware for installation on a pole. Instructions for siting are provided in the Atmosphere Investigation.
An electronic tipping bucket rain-measuring instrument may be used in conjunction with an automated weather station. The tipping bucket must have a resolution of at least 0.25 mm.
The depth of daily snowfall will be measured with a plywood board, painted white, which is approximately 40 cm X 40 cm x at least 1 cm thick.
The rain gauge described in Precipitation, Liquid will be used for this measurement.
For snow depths less than 1 meter, a meter stick is recommended. When the snow is deeper than one meter, a snow depth pole is used. This can be made from a 2 meter pole by placing two meter sticks end to end on this pole.
The same instruments described in Hydrology: Water pH will be used for this measurement.
A digital hygrometer or sensor must provide a digital readout of relative humidity to the nearest 1%. Over a range of 20-95%, accuracy must be at least 5%.
The digital hygrometer should include a stand to allow the unit to be placed upright on the floor of the instrument shelter, while measurements are being taken. Calibration is done by the manufacturer and should be warranted for at least two years, with subsequent recalibration available. Batteries should be included. The unit should not be left outside on a daily basis. In humid environments, storing the digital hygrometer with an absorbent desiccant is advised.
The wet bulb and dry bulb temperatures shall be measured with a sling psychrometer, which consists of two spirit-filled thermometers. The thermometers shall be readable only in degrees Celsius, with scales marked in increments of 1.0 ˚C, and the scales must be capable of supporting temperature estimations to the nearest 0.5 ˚C over a range of –1 ˚C to 35 ˚C. The psychrometer must be in a sturdy protective case or have spirit bulbs mounted on a rigid plate, and be provided with handle necessary for whirling or slinging. Thermometers must be factory calibrated to an accuracy of +1.0 °C, which will provide relative humidity accuracy of 5%. Both scales should be adjustable for calibration, or the spirit bulbs replaceable. Each scale must be clearly marked to indicate Celsius. Siting and installation instructions are provided in the Atmosphere Investigation.
The Calibration Thermometer described in Air Temperature may be used for this measurement.
The Maximum/Minimum Thermometer described in Air Temperature may be for this measurement.
The Instrument Shelter described in Air Temperature will be used for this measurement.
The ozone chemical strips contain a solution of tin(II) chloride dihydrate and 1,5-diphenylcarbazide dissolved in reagent-grade acetone. When exposed to air, ozone reacts with the mixture and triggers a colorimetric reaction resulting in the formation of a pink color. Ground level ozone concentrations can be measured by quantifying the color change on an exposed chemical strip using an ozone optical reader.
The ozone test strip optical reader operates as a simple spectrophotometer consisting of a light emitting diode (LED) emitting light near 540nm, and a photo diode that captures the reflected light off the exposed chemical test strip and converts it into an electrical voltage. The reader must be calibrated so that the voltage measured can be displayed as an ozone concentration in parts ozone per billion parts of air (ppb). Zero ozone level must be set by inserting an unexposed ozone test strip into the reader and storing the voltage produced. Any absorption at 540 nm above this value will be measured as a specific ozone concentration.
Directions for constructing an ozone measuring station are provided in the Instrument Construction: Surface Ozone in the Atmosphere Investigation.
Any device capable of displaying wind direction, such as weathervane. Directions for constructing a wind direction instrument are provided in the Instrument Construction: Surface Ozone in the Atmosphere Investigation.
The Infrared thermometer should be a handheld instrument. It must have an accuracy of +/-1 ˚C over a range of –32 ˚C to 72 ˚C.
The GLOBE/GIFTS water vapor instrument is based on the same principle as and similar in design to the GLOBE sun photometer, the specifications for which are described in detail under Aerosols. Both use light emitting diodes (LEDs) to measure the strength of sunlight in select wavelengths. While the GLOBE sun photometer detects visible light in the green and red part of the spectrum, the water vapor instrument detects infrared rather than visible light. This instrument concept was first developed and described in the scientific literature by a member of the Water Vapor protocol Science Team [Mims, Forrest M. III, Sun photometer with light-emitting diodes as spectrally selective detectors, Applied Optics, 31, 6965-6967, 1992].
The calibrations of the LEDs for this instrument require access to highly specialized equipment and data and they cannot be duplicated by students in the lab or in the field. These instruments can be obtained from the GLOBE Water Vapor Team.
A weather station must be attached to a data logger and computer, and be capable of logging data at 15-minute intervals. Data entry is simplified if the software for the weather station supports the option to “Export GLOBE Data”.
The sensors attached to the weather station must meet the following specifications:
Temperature: Must have either an accuracy of ± 0.5 ºC or a precision of at least ± 0.5 ºC and an error offset that is temperature independent.
Barometric Pressure: Must have a pressure range of between 940 and 1060 mbars with one mbar resolution and an accuracy of 3.5 mbars over its entire range.
Relative Humidity: Must have a digital readout of relative humidity to the nearest 1%. Over a humidity range of 20-95%, accuracy must be at least 5%.
Rainfall: Must have a resolution of at least 0.25 mm.
Anemometer: Must have a precision of ± 5% and a range of at least 0-34 m/s
You may report data taken using any sensors that meet these specifications. In order to perform a weather station protocol and related email data entry these sensors must be attached to a weather station that is capable of logging data at 15-minute intervals.
If one or more of the sensors of your weather station do not meet the above specifications, you may still report data collected with the sensors that do meet specifications.